For vehicles, and in particular heavy goods vehicles such as trucks, a gearbox, also called a range gearbox, is often connected to the main gearbox with the objective of doubling the number of potential gearings. Such an additional gearbox usually comprises a planetary gear, which has a low and a high gear, with which the gear possibilities of the main gearbox may be divided into a low range position and a high range position. In the low range position, a gear reduction occurs through the planetary gear, and in the high range position the gear ratio is 1:1 in the planetary gear.
The range gearbox is arranged between the main gearbox and a propeller shaft connected to the driving wheels of the vehicle. The range gearbox is housed in a gearbox housing and comprises an input shaft connected to the main gearbox, an output shaft and a planetary gear arranged between the input shaft and the output shaft. The planetary gear usually comprises three components, which are arranged in a rotatable manner in relation to each other, namely a sun wheel, a planetary wheel carrier with a planetary wheel and a ring gear. With knowledge about the number of teeth in the sun wheel and the ring gear, the mutual rotational speeds of the three components may be determined during operation. In a range gearbox, the sun wheel may be connected in a twist-fast manner with the input shaft, with a number of planetary wheels that engage with said sun wheel, which planetary wheels are mounted in a rotatable manner on the planetary wheel carrier, which is connected in a twist-fast manner with the output shaft, and with an axially shiftable ring gear, which envelops and engages with the planetary wheels. The teeth of the sun wheel, the planetary wheels and the ring gear may be oblique, i.e. they have an angle in relation to the rotational axis common to the sun wheel, the planetary wheel carrier and the ring gear. By cutting the teeth obliquely, the noise emitted from the planetary gear is reduced. However, a reaction force in the direction of the rotational axis is obtained from the cogwheels comprised in the planetary gear. The direction of the reaction force is dependent on the direction in which the cogwheels in the planetary gear are obliquely cut. Thus, the reaction forces may operate backwards or forwards along the extension of the rotational axis.
In a prior art range gearbox, low and high gears are achieved by axial sliding of the ring gear between the low range position, in which the ring gear is rotation-locked in relation to the gearbox housing, and the high range position, in which the ring gear is rotatable in relation to the gearbox housing, and where the ring gear, the planetary wheels and the sun wheel rotate as one continuous unit. The planetary gear comprises two coupling rings arranged on each side of the ring gear, and two synchronisation rings arranged on each side of the ring gear. The synchronisation rings are adapted to achieve synchronous gear shifting.
In order to achieve a good synchronising function in this type of range gearbox, the surface of the synchronisation ring's teeth, which faces the ring gear and which is intended to receive the ring gear's teeth on synchronisation, must have an angle—a so-called locking angle—in relation to the synchronisation ring's rotational axis, which locking angle must be balanced against the braking torque that the synchronisation ring transfers to the ring gear, in order to achieve a synchronous rotational speed. This means that said locking angle must be adapted so that the teeth on the synchronisation ring abut against those parts of the ring gears' teeth that are equipped with a locking angle, and so that they impact sufficiently on the ring gear in order for a synchronous rotational speed to be achieved, and so that they, subsequently, are released from those parts of the ring gear's teeth that are equipped with the locking angle, which happens when the ring gear is to engage with the relevant coupling ring, after the synchronous rotational speed has been achieved. In order to secure that a synchronous rotational speed is obtained before the ring gear goes past the synchronisation ring in an axial direction, the teeth of the synchronisation ring must not let go of the ring gear's teeth too easily.
After the synchronisation ring's teeth have been released from the ring gear's teeth, when a synchronous rotational speed has been achieved between the ring gear and the coupling ring, the ring gear will be shifted axially, so that the synchronisation ring is inserted in the ring gear and remains in an axial position in relation to the ring gear, which axial position is determined by the position where the synchronisation ring meets and abuts against the planetary wheels of the planetary gear.
The ring gear's freedom of movement in an axial direction is limited by the geometrical design of the teeth of the ring gear and the coupling ring. At the ring gear's axial ends, end surfaces at the tips of the ring gear's teeth meet and abut a circumferential end surface of each coupling ring, which entails that the ring gear may not be shifted in an axial direction.
The document WO0155620 shows a synchronisation device at a planetary gear, wherein the planetary gear comprises a sun wheel, a planetary wheel carrier and a ring gear. The sun wheel is connected in a twist-fast manner with the input shaft, and a number of planetary wheels engage with the sun wheel, which planetary wheels are rotatably mounted on a planetary wheel carrier, connected in a twist-fast manner with an output shaft. An axially shiftable ring gear envelops and engages with the planetary wheels. The low and high gears of the gearbox are obtained by the ring gear being shifted axially between the low range position and the high range position.
These synchronisation devices are, however, subjected to wear and result in considerable costs of repair. If the range gearbox transfers large torques, the synchronisation devices will have considerable dimensions, entailing increased weight, increased space requirement and increased moment of inertia. When the high range position is connected, the torque will be transferred from the sun wheel to the planetary wheels, entailing that facets may form on the cog flanks of the sun wheel, which creates noise in the gearbox and accelerates wear of the planetary gear's cogwheels.
There are range gearboxes, in which the synchronisation devices are replaced with splined coupling sleeves. By controlling the transmission in such a way that a synchronous rotational speed arises between the two components that are to be connected, an axial shift of the coupling sleeve is facilitated along the two components, with the objective of connecting and interconnecting these. When the components are to be disconnected from each other, the transmission is controlled in such a way that torque balance arises between the components, meaning that the coupling sleeve does not transfer any torque. It then becomes possible to displace the coupling sleeve along the components with the objective of disconnecting them from each other.
Torque balance relates to a state where a torque acts on an internal ring gear arranged in the planetary gear, representing the product of the torque acting on the planetary wheel carrier of the planetary gear and the gear ratio of the planetary gear, while simultaneously a torque acts on the planetary gear's sun wheel, representing the product of the torque acting on the planetary wheel carrier and (1—the planetary gear's gear ratio). In the event two of the planetary gear's component parts, i.e. the sun wheel, the internal ring gear or planetary wheel carriers, are connected with a clutch device, this clutch device does not transfer any torque between the planetary gear's parts when torque balance prevails. Accordingly, the clutch device may easily be shifted and the planetary gear's component parts may be disconnected.
The document U.S. Pat. No. 6,196,944 shows a planetary gearbox, comprising a sun wheel, a planetary wheel carrier with planetary wheels and a ring gear. The sun wheel may, via a coupling sleeve, be connected in a twist-fast manner with the input shaft in a low range position, and be disconnected from the input shaft in a high range position. In the high range position, the input shaft is connected with the planetary wheel carrier via the same coupling sleeve. The ring gear is fixedly connected with a gearbox house. The prior art planetary gear is arranged inside a supplemental gearbox, which has only two gear positions.
The reverse gear in a transmission of a vehicle is often placed in the main gearbox, which then comprises a cogwheel, which is connected when the vehicle has to reverse. The cogwheel, which is intended for the reverse gear, entails an extension of the main gearbox and an undesired weight increase of the vehicle. The cogwheel rotates in the opposite direction to other cogwheels in the main gearbox, which entails losses. Said cogwheel intended for the reverse gear often has a tendency to induce unwanted rattle in the transmission, arising from an intermediate cogwheel, arranged between a countershaft and the main shaft in the main gearbox.
Prior art gearboxes must often be completely dismantled when a component is replaced, which means that it takes a lot of time and that it is costly to repair the gearbox.